In weighing systems generally, it is ordinarily desirable or essential that the output indication of the weighing system indicate a predetermined reference value, ordinarily zero, when no object is being weighed, so that the change in the output indication produced by an object being weighed will accurately represent the true weight of the object. While it is usually easy to construct and adjust a weighing system initially to provide the desired output indication of zero when zero weight is being applied, the zero reference usually tends to change somewhat with time, usually randomly and unpredictably, due to factors such as changes in the system components caused by aging or ambient temperature change or the like, or such changes in the zero reference may occur because of inadvertent changes in the tare weight of the system, for example due to undersired accumulation of foreign matter on the weight-receiving platform or its supports. Any such changes in the zero reference setting will produce corresponding errors in the indicated weight of the object.
It is therefore common to provide a manually-operable zero-set control in weighing systems, so that when an operator observes that the ouput indication is not zero under steadystate conditions between successive weighings, he can readily adjust the manual control until the output indication becomes zero again under these conditions. However, this requires that an operator observe the discrepancy in zero setting, and interrupt the weighing long enough to re-set the zero as accurately as his visual acuity, manual dexterity and carefulness permit.
The present invention is particularly concerned with, and will be described with special reference to, the general type of automatic electronic weighing system which is described and claimed in U.S. Pat. No. 3,800,893 of J. D. Ramsay and G. R. Weaver, filed Sept. 5, 1972 and issued Apr. 2, 1974, and more recent modifications thereof. In such systems, successive objects to be weighed are moved sequentially and rapidly onto and from the weighing platform of a spring-restrained balancebeam type of weigh cell, and three separate electrical signals are derived representing, respectively, the instantaneous displacement, velocity and acceleration of the weighing platform and object produced in response to the weight of that object. These three signals are fed into a circuit, which, in effect, solves the second-order differential equation of motion of the platform and object, to produce an output signal representing the weight of the object. With this arrangement, it is not necessary to wait for oscillations of the weigh cell to die out in order to obtain a proper weight indication, and in fact weight indications are commonly obtained by such sensing and measuring of the platform motion during a cycle or two, or even a fraction of a cycle, of its oscillation. Because of this, weighing can be accomplished in an extremely short time, even with a lightly damped weigh cell, and hundreds of objects are readily weighed per minute.
In the weighing system of the above-cited patent, the electrical circuit which solves the second-order differential equation of motion, herein designated as the resolver circuit, produces an electrical analog output in the form of a current or voltage the level of which, during weighing time intervals in which the weight is on the platform and the system in a stable oscillatory condition, represents the object weight. During each such interval, a time-controlled analog integrator responds to the resolver output signal to produce an output signal which is representative of the integrated value of the output signal during such interval, and is used as an indication of object weight.
Since the development of the above-described analog weighing system of the cited patent, a modification of the above-described time-controlled analog integrator has been developed, employing instead a digital integrator which produces output pulses at a rate proportional to the instantaneous level of the output signal from the resolver, and a time-controlled BCD counter which is controlled to count the number of such pulses occurring during the desired weighing time interval for each object to be weighed. The successive counts thereby accumulated in the binary counter represent the weights of the successive objects being weighed. Such successive digital weight indications may be visually displayed, printed out, or used for a variety of control functions. In one specific application in which the weighing system is to be used for the purpose of sorting out filled food containers which are underweight, the binary counter output may be supplied to a comparator, which produces an output control signal whenever the indicated weight is less than the predetermined desired weight, and operates a diverter for removing such underweight containers from the train of objects being weighed; various alarm devices may also be automatically operated under such condition. The times during which the binary counter operates to perform its counting function may be initiated by a start pulse produced in response to photoelectric or other detection of the arrival of the object to be weighed into its proper weighing position, the count being terminated a predetermined time interval later.
In both the analog and digital systems described above, the analog output of the resolver circuit is intended to remain at a constant reference value, i.e. zero, when no object to be weighed is positioned on the weigh cell. To accomplish this, a controllable DC zero-reference correction voltage is preferably generated and substracted from the input displacement signal to the resolver, and manually adjusted in the absence of an object in weighing position so that the resolver circuit then exhibits zero output. If departures of the resolver output voltage from zero reference value are observed, the zero reference voltage at the resolver input can be manually adjusted to re-establish the desired zero setting.
It is also known to be possible to accomplish automatic zero correction by sensing the analog output voltage of the resolver and feeding it back in degenerative polarity to the input to the resolver during non-weighing intervals, thereby automatically to reduce the resolver output voltage to the desired zero reference value; in such a system, an analog hold circuit would be included in the negative feedback path, the function of which is to hold the feedback correction voltage from the end of one such zero-correction time, through one or more intervening weighing intervals, until a later non-weighing correction interval.
The present invention is concerned with an improved electronic automatic zero correction apparatus adapted for use in the type of automatic weighing system employing a digital integrator, which is simple, accurate, compact, and not subject to substantial overshoot of the correcting signal, nor to drift of correction voltage in the weighing intervals between correction times, and which provides zero correction for changes in operating characteristics of the circuit up through the output of the digital integrator, and not merely up to the output of the resolver circuit.